This invention relates to an improvement in large insulated containers for shipboard transportation of cryogenic liquified gas, such as liquified natural gas, at low or near atmospheric pressure. It has application particularly to single-walled metallic thin-shell type containers which do not require extensive internal stiffening to function satisfactorily. Such containers are supported directly in the ship by the simple expedient of sitting upon and being secured to the inner bottom of the ship's cargo holds, thus transferring the principal components of the cargo loading directly to the ship's structure in the most effective location via load-bearing insulation on the innerbottom.
The safe and efficient transportation of liquified gas, such as liquified methane at -260.degree. F, presents special problems in the design of the containment system of the ship. Ordinary shipbuilding materials used in the hull structure suffer severe embrittlement at these temperatures, and an almost immediate brittle fracture of some of the hull structure would occur should the liquified cargo come in contact with it. Containers, or some kind of containment system, must therefore be used to contain the liquid cargo and keep it out of contact with the ship's hull. The containers must be made from special material suitable for the cryogenic temperature, such as, for example, aluminum alloy, 9% nickel steel, or 18-8 chromium-nickel stainless steel, which are costly to buy and fabricate. In addition, the containers must be supported in the ship in a manner to permit the significant thermal contraction of the container which will occur on cooling down when loading, and to minimize the interactions between the ship's structure and the tank itself due to cargo loading and ship response to wind and waves. Also, the containers must obviously be surrounded by an external jacket of insulation of sufficient thermal resistance to keep the temperature of the surrounding ship's structure from falling below the permissable transition temperature for satisfactory ductility for the type of structural steel used in the ship.
Two methods have commonly been used to accomplish the containment discussed above. One is to construct the cargo containment tanks of sufficient strength and rigidity to carry the liquid load independently, without assistance from the ship's structure except for support derived from some supporting apparatus or mounting system. The other is to construct the cargo containment of light or thin material not intended to be loaded appreciably in its own plane, but which contains the cargo by transmitting the liquid pressure perpendicular to the thin material to the structure of the ship's hold via the insulation. The former structures are usually referred to as independent, or self-supporting tank systems, while the latter are referred to as integrated, or membrane systems. This invention is exclusively concerned with a new concept of octangular curvilinear containers which is of the independent and self-supporting category, except for the light flat bottom portion of the container which derives support from the load-bearing insulation on the ship's innerbottom, and which may properly be called semi-membrane.
Large dimension flat bottomed tanks, such as those that are the subject of this invention, are best adapted to containing liquids where the gravity loading is the more important part of the cargo loading, and the internal back pressure on the surface of the liquid is low. Thus, they are well adapted to contain liquified natural gas which is normally stored at near atmospheric pressure, typically 11/2 to 21/2 psig, where such conditions apply. While there exists a considerable background of experience and technology for their use from the extensive application of flat bottomed cylindrical tanks to the storage of LNG ashore, such tanks must be modified and adapted significantly to meet the particular conditions and requirements aboard ship. In particular, whereas the designer of a shore tank usually has rather free use of the surrounding space so that he may proportion the tank as he sees fit to minimize construction and material costs, the designer of a shipboard container faces rather severe spatial constraints and must give full consideration to selecting proportions and a geometric shape which will result in an efficient and wholesome ship design as well as an economical and safe tank design. A primary purpose of the invention is, therefore, to provide an improved bottom-sitting container which has a geometric shape well suited to the usual range of ship proportions and cargo hold configurations of normal liquid bulk carrying ships, and to meet the operating and design requirements for the ship, as well as for the container, so as to result in an efficient and wholesome liquid gas tanker.